<?php
date_default_timezone_set('Europe/Sofia');

$today = new DateTime();

$day = $today->format('d');
$month = $today->format('m');
$year = $today->format('Y');

$firstDateOfTheCurrentMonth = new DateTime("01" . "-" . $month . "-" . $year);
$lastDayOfTheCurrentMonth = cal_days_in_month(CAL_GREGORIAN, $month, $year);
$lastDateOfTheCurrentMonth = new DateTime($lastDayOfTheCurrentMonth . "-" . $month . "-" . $year);

while ($firstDateOfTheCurrentMonth <= $lastDateOfTheCurrentMonth) {
    if ($firstDateOfTheCurrentMonth->format('l') === Sunday) {
        echo $firstDateOfTheCurrentMonth->format('d-m-Y') . "\n";
    }

    // go to the next day
    $firstDateOfTheCurrentMonth->add(new DateInterval('P1D'));
}